Emergence and Effective Field Theories

In a wide range of areas in physics, including condensed matter physics, cosmology, high energy physics, string theory and quantum gravity, a lot of emphasis is put on the notion of effective field theories. This workshop focuses on what is common among these uses of effective field theory and in particular on the meaning of claims made using them that various features of the physical world are "emergent." What is sometimes meant by "emergence" is that there are macroscopic phenomena whose description requires concepts and principles that are not part of the description of the fundamental degrees of freedom. This may sound simple, but lurking behind it are a minefield of conceptual and technical issues that have led to confusion among both physicists and philosophers. The aim of this workshop is to bring together physicists from the several areas where effective field theories are employed with a small number of philosophers who have been investigating methodological issues related to making sense of the concept of emergence.

This talk considers the extent to which the intertheoretic relation between an EFT and its (possibly hypothetical) high-energy theory supports a notion of emergence. When a high-energy theory exists, this relation is based on a process that involves the elimination of high-energy degrees of freedom. This elimination results in an EFT that formally bears little resemblance to the high-energy theory. I investigate the extent to which this lack of formal resemblance underwrites notions of novelty and autonomy that may be appropriately associated with emergence.

I'll begin by reviewing the method by which an EFT is constructed from a high-energy theory by means of integrating out high-energy degrees of freedom from the latter. I'll then review a number of attempts in the philosophical literature to explicate the notion of emergence. I'll first consider general phillosophical accounts that identify emergence as supervienience without reduction, or as associated with various notions of autonomy (reductive, predictive, causal, and/or explanatory). I'll then consider more specific accounts related to physics in particular, including Batterman's (2002) notion of the failure of a limiting relation, and Mainwood's (2006) description of the concept of emergence associated with the claims of condensed matter physicists (e.g., Anderson 1972). This account conceives emergence as microphysicalism (the claim that emergent properties/entities are ultimately composed of microphysical properties/entities) coupled with novelty cashed out in terms of a mechanism (in this case spontaneous symmetry breaking) that produces a reduced phase space supporting (emergent) properties that are not explicitly defined on the initial phase space. A similar account is given by Wilson (2010), who explicates novelty in terms of an elimination of degrees of freedom. I'll suggest that Batterman's account does not quite succeed in the context of EFTs (simply put, the relation between an EFT and its high-energy theory cannot be described in terms of the failure of a limiting relation), and while the elimination of degrees of freedom does occur in EFTs, this process is different from the process described by Mainwood and Wilson (in particular, the phase space of an EFT is not, in general, a reduced phase space of a high-energy theory). This suggests that a notion of emergence as microphysicalism coupled with novelty can be applicable to the EFT context, as long as an appropriate mechanism that underwrites novelty, other than spontaneous symmetry breaking, can be identified. This mechanism perhaps can be identified simply as the particular approximation scheme employed in the construction of an EFT.

Ganapathy Baskaran, Institute of Mathematical Sciences

More is Different in the Quantum World, in its Own Way

In his article `More is Different', P W Anderson (1972) sensitized the physics community about importance of emergence, by using concepts such as broken symmetry, emergent hierarchical structures, constructionists converse of reductionism etc. The manifestation of complexity and hierarchy in the quantum many particle systems go beyond broken symmetries. In certain quantum systems we have the opposite - emergence of new local gauge symmetries. This idea was introduced, for example, for Mott insulators, by Anderson and us in 1988. Properties such as entanglement, different possible statistics of identical particles, multi particle wave interference, vastness of Hilbert space etc;, which are unique to the world of quantum, leads to a wonderful richness in physics and mathematics, including emergent gauge symmetries, gauge fields, quantum order, holographic correspondences and continuing surprises.

Robert Batterman, University of Pittsburgh

The Tyranny of Scales

How can one model the behavior of materials that display radically different, dominant behaviors at different length scales. Although we have good models for material behaviors at small and large scales, it is often hard to relate these scale-based models to one another. Macroscale (effective) models represent the integrated effects of very subtle factors that are practically invisible at the smallest, atomic, scales. For this reason it has been notoriously difficult to model realistic materials with a simple bottom-up-from-the-atoms strategy. The widespread failure of that strategy forced physicists interested in overall macro-behavior of materials toward completely top-down modeling strategies familiar from traditional continuum mechanics. The problem of the "tyranny of scales" asks whether we can exploit our rather rich knowledge of intermediate micro- (or meso-) scale behaviors in a manner that would allow us to bridge between these two dominant methodologies. Macroscopic scale behaviors often fall into large common classes of behaviors such as the class of isotropic elastic solids, characterized by two phenomenological parameters---so-called elastic coefficients. Can we employ knowledge of lower scale behaviors to understand this universality---to determine the coefficients and to group the systems into classes exhibiting similar behavior?

Gordon Belot, University of Michigan

Arguments for the Emergence of Spacetime Topology

It is widely held that string theory shows that spacetime geometry and topology are emergent rather than fundamental. Often it is said that this follows from the various interesting dualities that exist within string theory. I will discuss the argument from duality, contrasting it with older arguments for the non-objectivity of spatiotemporal topology. I hope that this will clarify some questions about the role of spacetime in string theory---and about the differences between the ways that philosophers and physicists approach these questions.

Yves Couder, Ecole Normale Superieure

A Macroscopic-scale Wave-particle Duality : the Role of a Wave Mediated Path Memory

It is usually assumed that the quantum wave-particle duality can have no counterpart in classical physics. We were driven into revisiting this question when we found that a droplet bouncing on a vibrated bath could couple to the surface wave it excites. It thus becomes a self-propelled "walker", a symbiotic object formed by the droplet and its associated wave.

Through several experiments, we addressed one central question. How can a continuous and spatially extended wave have a common dynamics with a localized and discrete droplet? Surprisingly, quantum-like behaviors emerge; both a form of uncertainty and a form of quantization are observed. This is interesting because the probabilistic aspects of quantum mechanics are often said to be intrinsic and to have no possible relation with underlying unresolved dynamical phenomena. In our experiment we find probabilistic behaviors and they do have a relation with chaotic individual trajectories. These quantum like properties are related in our system to the non-locality of a walker that we called its "wave mediated path memory". The relation of this experiment with the pilot wave model proposed for quantum mechanics by de Broglie will be discussed.

In the philosophical literature, effective field theories have been regarded as emergent in the sense of furnishing novel explanations. In particular, Batterman has argued that effective field theories in statistical mechanics are emergent in this sense. I will argue that effective field theories in quantum field theory do not furnish analogous novel explanations. There are relevant disanalogies between statistical mechanics and quantum field theory with regard to the roles played by idealizations and the explanatory goals of the application of renormalization group methods. Contrasting the statistical mechanics and quantum field theory cases highlights the role that the physical interpretation of the formalism and the goals of theorizing play in determining whether a particular effective theory counts as emergent.

Nigel Goldenfeld, University of Illinois

Emergence and Minimal Models in Condensed Matter Physics and Biology

Our ability to understand the physical world has to a large extent depended on the existence of emergent properties, and the separation of scales that permits effective field theory descriptions to be useful. Exploiting this fact, we can construct minimal models that enable efficient calculation of desired quantities, as long as they are insensitive to microscopic details. This works in many instances in physics, and I give some examples drawn from the kinetics of phase transitions mediated by topological defects. In other fields, such as biology, it is not so clear that these concepts are useful, and I will discuss to what extent emergence and effective theories might be useful.

Alioscia Hamma, Perimeter Institute

Entanglement and the Emergence of Thermalization

The canonical example of emergence is how thermodynamics emerges from microscopic laws through statistical mechanics. One of the vexing questions in the foundations of statistical mechanics is though how is it possible to justify thermalization in a closed system. In quantum statistical mechanics, entanglement can give the key to answer this question, provided that they are typically very entangled. Fortunately, most states in the Hilbert space are maximally entangled. Unfortunately, most states in the Hilbert space of a quantum many body system are not physically accessible. We show that the typical entanglement in physical ensembles of states is still very high.

Ted Jacobson, University of Maryland

Can Lorentz Symmetry be Emergent?

I will begin by discussing some of the strongest observational evidence for Lorentz symmetry, and the essential role that Lorentz symmetry appears to play in the consistency of black hole thermodynamics. Next I will discuss some reasons for suspecting that Lorentz symmetry may nevertheless be emergent. And finally I will discuss difficulties with the concept of emergent Lorentz symmetry, and how such difficulties might conceivably be overcome.

Leo Kadanoff, The James Franck Institute University of Chicago

Is the Renormalization Group Really That Ugly?

P.A.M. Dirac has famously said that he would like the renormalization method better if it were not so ugly. In this talk, I shall indicate that the method is used to construct connections among theories in different domains of size. Further I shall argue that physics should be mostly about bridging different domains of theory. I shall comment upon possible differences in attitudes toward bridging held by condensed matter physicists and by particle physicists.

Margaret Morrison, University of Toronto

Why is More Different?

Emergent phenomena are typically described as those that cannot be reduced, explained nor predicted from their microphysical base. However, this characterization can be fully satisfied on purely epistemological grounds, leaving open the possibility that emergence may simply point to a gap in our knowledge of these phenomena. By contrast, Anderson’s (1972) claim that the whole in not only greater than but very “different from” its parts implies a strong ontological dimension to emergence, one that requires us to explain how, for example, superconductivity can be ontologically distinct from its micro-ontology of Cooper pairing. This is partly explained by using RG methods to show how the ‘universal’ characteristics of emergent phenomena are insensitive to the Hamiltonian(s) governing the microphysics. But this is not wholly sufficient since it is possible to claim that the independence simply reflects the fact that different ‘levels’ are appropriate when explaining physical behavior, e.g. we needn’t appeal to micro properties in explaining fluid behavior. The paper attempts a resolution to the problem of ontological independence by highlighting the role of spontaneous symmetry breaking in the emergence of universal properties like infinite conductivity. If we focus on the dynamical aspects of symmetry breaking rather than interpreting it as an organizing principle (Laughlin and Pines, 2000) we are able to see how it, together with the RG arguments, illustrates both how and why emergent phenomena can be considered different from their micro-constituents.

Laura Ruetsche, University of Michigan

Take it to the Thermodynamic Limit

Prominent philosophers of physics, including Craig Callender and John Earman, have issued stern warnings against drawing any foundations of physics conclusions from theories obtained by taking the thermodynamic limit. Without dismissing these worries entirely, I argue that we shouldn't take them too seriously.

Ramamurti Shankar, Yale University

Emergent Lorentz Invariance in Superconductors and its Novel Consequences

Does Time Emerge from Timeless Laws, or do Laws of Nature Emerge in Time?

The history of physics, from Galileo's spatialization of time to Einstein's block universe, and on to Julian Barbour's timeless quantum cosmology, tells a story by which time is demoted from a fundamental aspect of experience to an emergent illusion in a world held to be fundamentally timeless. The question I would like to address is, is this correct, or will the next stage in the development of physics require a rediscovery of time as a primary aspect of nature?

One reason to bet on the reality of time is the strength of the argument of Pierce that laws of nature require explanation and that laws must evolve to be explained. This implies that time is prior to law, which means time cannot emerge from timeless law. This however raises a problem: is the evolution of law lawful? Are all laws effective and approximate? Is there a metalaw which governs the evolution of laws? If so, what selects the metalaw?

One approach to this meta-laws dilemma is that the distinction between dynamical laws and the states they act on, which is absolute in most physical theories, is emergent. I present a simple model to illustrate this approach.

This talk is partly based on joint work with Roberto Mangabeira Unger.

Philip Stamp, University of British Columbia

Decoherence and Effective Field Theories

Effective field theories, underpinned by the resnormalization framework, are a central feature of condensed matter physics and relativistic field theory. However the phenomenon of decoherence is not so easily subsumed under this framework. Ordinary environmental decoherence may lead to very unusual effective theories, and recent ideas about intrinsic decoherence in Nature (eg., Penrose's ideas aobut gravitational decoherence) do not obviously lead to any effective field theory. I will review our ideas aobut environmental decoherence, with some examples from condensed matter physics, highlighting some of the peculiar features of these. I will then discuss what we know of intrinsic decoherence (which in some cases amounts to a breakdonw of quantum mechanics, focussing on a new path integral formulation of Penrose's ideas.

Bill Unruh, University of British Columbia

Emergence/analogy and Hawking Radiation

The concepts of emergence and analogy are very closely related -- A is like B vs A is B. I will discuss this in the context of the emergence of/analogy with Hwking radiation in the arena of fluid systems, and the possibility of doing experiments in the lab. Does this mean gravity is emergent from some aether like theory? I think attempts to do that are fraught with difficulties, and will briefly discuss why I think so.

Andrew Wayne, University of Guelph

Emergence and Effective Field Theories in Gravitational Physics

This paper has two aims. The first is to improve upon the diverse and often muddled philosophical characterizations of emergence by articulating reasonably precise necessary and sufficient conditions for a phenomenon to count as emergent in physics. Central to this account of emergence is the idea that emergent phenomena cannot be explained reductively. The second aim of the paper is to apply this account to the use of effective field theories in gravitational physics. Effective field theories have recently been applied to model the inspiral trajectories (and other features) of two compact, massive objects orbiting each other, with excellent predictive success. The calculational machinery has been ported from quantum field theory, but the physical interpretation is significantly different. The paper concludes that this application of effective field theories to gravitational physics is clearly not a case of emergence.

Silke Weinfurtner, University of British Columbia

Quantum Gravity Laboratory

At the level of effective field theory it is possible to establish analogies between non-gravitational and gravitational systems. For example, first order perturbation equations in an analogue gravity model can be written as a wave equation in a curved spacetime. Perhaps the most intriguing application of analogue gravity systems is the possibility to experimentally investigate open questions in semi-classical quantum gravity, such as the black hole evaporation process. I will briefly discuss our recent black hole experiment, which demonstrates the universality of the Hawking process. If time permits, I will discuss the possibility to extend the analogue gravity programme, and outline the necessary steps towards full quantum gravity experiments.